average heart rate

Question regarding the average heart rate for a workout reported in the BT logs...

I did a long run last night and used my 305 to record average HR every half mile.  I then uploaded the lap data to the BT log.  The average heart rate reported by the BT log is 158, but the average HR reported by my Garmin is 149.  The average HR I get by averaging the average HR reported for each lap in the BT log is also 149.

Any idea why the average HR for the workout calculated by the BT log is so different from the actual value (158 vs 149)?

I've also noticed this discrepancy for other runs, but usually it's not this large.

Thanks.